Phrasing is pretty important here. Thinking of the problem as why does shuffling an array make it slower gives a hint as to the root cause.
This is one of those times when I wish people could favorite or make their own personal link lists on lobsters. With social bookmarking mostly dead, sites like this are the only way people can share and keep track of new things posted online. Right now I have to save things in a completely different site/app and then link back to the topic of conversation in a note. :/